In general, in a large power system requiring large-capacity and long-distance power transmission, high voltage transmission is necessary to increase a transmission voltage in terms of a reduction of power loss, a construction site problem, and an increase in power transmission capacity.
Power transmission methods may be largely classified into an alternating-current (AC) power transmission method and a direct-current (DC) power transmission method. The DC power transmission method refers to transmission of power by direct current. Specifically, in the DC power transmission method, first, a power transmission side converts AC power into an appropriate voltage, converts the voltage into direct current by a converter, and transmits the direct current to a power reception side via a transmission line, and the power reception side converts the direct current into AC power by an inverter.
In particular, the DC transmission method has been widely used because this method is advantageous in transmitting a large amount of power over a long distance and can be operated in connection with an asynchronous power system, and a loss rate of direct current is low and a stability thereof is high in long-distance transmission, compared to alternating current.
An insulator of a (ultra) high-voltage DC transmission cable used in the DC transmission method may be formed of insulating paper impregnated with an insulating oil or an insulating composition containing a polyolefin resin as a base resin. Recently, as cables are likely to be operated at relatively high temperatures, an insulator formed of an insulating composition containing a polyolefin resin that increases the amount of power transmission and is free from leakage of an insulating oil have come into widespread use.
However, the polyolefin resin has a linear molecular chain structure and thus is applied to an insulating layer of a cable by improving mechanical and thermal properties of the polyolefin resin by a crosslinking process. Thus, space charges are likely to be accumulated in the insulating layer of the cable due to crosslinking by-products inevitably generated due to decomposition of a crosslinking agent during the crosslinking process. The space charges may distort an electric field in the insulator of the (ultra) high-voltage DC transmission cable and thus dielectric breakdown may occur at a voltage lower than an initially designed breakdown voltage.
Such (ultra) high-voltage DC transmission cables are installed by connecting them to each other in units of tens to hundreds of meters via an intermediate connection part. Similarly, space charges may be accumulated in an insulating material of the intermediate connection part due to crosslinking by-products inevitably generated due to decomposition of a crosslinking agent during a crosslinking process. Accordingly, an electric field in the insulating material may distort and thus dielectric breakdown may occur at a voltage lower than an initially designed breakdown voltage of the intermediate connection part.
In the case of a cable used for a current-type DC transmission (LCC) that requires polarity inversion to change a direction of power transmission, an inorganic additive such as a magnesium oxide is uniformly dispersed in an insulating layer to solve the above problem. The inorganic additive is polarized in a DC electric field and thus space charges may be trapped, thereby minimizing electric field distortion caused by the accumulation of space charges. However, in the case of voltage-type DC transmission (VSC), polarity inversion is unnecessary, and an insulating composition to which an organic additive is added is used to optimize electrical stress to be applied to an insulator of the cable. Thus, it is necessary to accurately control the amount of space charges in the insulating layer.
Accordingly, there is an urgent need for an intermediate connection system, for an ultra-high-voltage DC power cable, which is capable of simultaneously preventing or minimizing electric field distortion, a decrease of DC dielectric strength, and a decrease of impulse breakdown strength due to the accumulation of space charges in an insulating layer of a cable or an insulating material of an intermediate connection part.